FLORIDA MARINE RESEARCH INSTITUTE TECHNICAL REPORTS. Scarring of Florida s Seagrasses: Assessment and Management Options

Transcription

1 FLORIDA MARINE RESEARCH INSTITUTE TECHNICAL REPORTS Scarring of Florida s Seagrasses: Assessment and Management Options F.J. Sargent, T.J. Leary, D.W. Crewz and C.R. Kruer Florida Department of Environmental Protection FMRI Technical Report TR

2 Lawton Chiles Governor of Florida Florida Department of Environmental Protection Virginia B. Wetherell Secretary The Florida Marine Research Institute (FMRI) is a bureau of the Florida Department of Environmental Protection (FDEP). The FDEP s mission is to protect, conserve, and manage Florida s environment and natural resources. The FMRI conducts applied research pertinent to managing marine-fishery resources and marine species of special concern in Florida. Programs at the FMRI focus on resource-management topics such as managing gamefish and shellfish populations, restoring depleted fish stocks and the habitats that support them, protecting coral reefs, preventing and mitigating oil-spill damage, protecting endangered and threatened species, and managing coastal-resource information. The FMRI publishes three series: Memoirs of the Hourglass Cruises, Florida Marine Research Publications, and FMRI Technical Reports. FMRI Technical Reports contain information relevant to immediate resource-management needs. Kenneth D. Haddad, Chief of Research Institute Editors Theresa M. Bert, David K. Camp, Paul R. Carlson, Mark M. Leiby, William G. Lyons, Anne B. Meylan, Robert G. Muller, James F. Quinn, Jr., Ruth O. Reese, Carmelo R. Tomas Judith G. Leiby, Copy Editor

3 Scarring of Florida s Seagrasses: Assessment and Management Options Frank J. Sargent, Timothy J. Leary, David W. Crewz Florida Department of Environmental Protection Florida Marine Research Institute St. Petersburg, FL and Curtis R. Kruer Consulting Biologist Summerland Key, FL Florida Department of Environmental Protection FMRI Technical Report TR

4 Cover Photograph Northwest of Windley Key in the Florida Keys. Photograph by Curtis R. Kruer, Copies of this document may be obtained from Florida Marine Research Institute 100 Eighth Ave. SE St. Petersburg, FL Attn: Librarian Document Citation Sargent, F.J., T.J. Leary, D.W. Crewz, and C.R. Kruer Scarring of Florida s seagrasses: assessment and management options. FMRI Tech. Rep. TR-1. Florida Marine Research Institute, St. Petersburg, Florida. 37 p. plus appendices. Document Production This document was designed in Microsoft Word (v. 5.1a) and formatted using Quark XPress (v. 3.3) on Apple Macintosh computers. Figures in Appendix B were created on a Sun Sparcstation 20 and exported to Adobe Illustrator (v. 5.5). Heading fonts are Adobe Avant Garde, and body text is Adobe Palatino. The cover headline is Adobe Gill Sans. The body paper is Finch Casablanca Opaque recycled, and the cover is Finch Fine. The document was designed by the authors, and McShane and Moore Communications, Inc. performed layout, graphics scans, and production for final film. Ralard Printers, Inc., printed the document. The text paper used in this publication meets the minimum requirements of the American National Standard for Permanence of Paper for Printed Library Materials Z

5 Table of Contents LIST OF TABLES... ii LIST OF FIGURES... iii ACKNOWLEDGMENTS... iv EXECUTIVE SUMMARY... v INTRODUCTION... 1 Seagrass Recovery... 5 Study Objectives... 7 METHODS... 7 Scarring Recognition... 9 Scar Mapping RESULTS DISCUSSION MANAGEMENT OPTIONS Four-Point Approach Education Channel Marking (Aids to Navigation) Enforcement Limited-Motoring Zones CONCLUDING REMARKS LITERATURE CITED APPENDIX A Methodology for Analyzing Scar Data MRGIS Integration Creating a Statewide Seagrass Coverage Error Reduction APPENDIX B... 45

6 List of Tables Table 1. NOAA/USGS charts...13 Table 2. Acreage of scarred seagrasses by county...14 Table 3. Relative percentage of scarred seagrasses by county...15 Table 4. Table 5. Within-county percentage of scarred seagrasses...17 Ranking of scarred-seagrass acreage by county...18 Table 6. Components of seagrass acreage in Monroe and Dade counties...19 Table 7. Scarred-seagrass acreage by region...19 Table 8. Moderately scarred sites in lower Florida Keys...25 Table 9. Severely scarred sites in lower Florida Keys...26 Table 10. Vessel registrations by county...27 Table 11. Vessel registrations by region...28 APPENDIX A Appendix Table 1. Sources of seagrass data...42 ii

7 List of Figures Figure 1. Seagrass species of Florida... 2 Figure 2. Aerial view of scarred seagrasses Lignumvitae Key, Florida Keys... 4 Figure 3. Seagrass rhizome differentiation... 5 Figure 4. Study area for assessment of seagrass scarring... 8 Figure 5. Example of polygon delineation Figure 6. Diagrams of scarring-intensity categories Figure 7. Recognition of scarring intensity Figure 8. Regions of Florida analyzed for scarred seagrasses Figure 9. Detailed map of scarred seagrasses Pine Island, Lee County Figure 10. Detailed map of scarred seagrasses Windley Key, Monroe County Figure 11. Example of channels ending in shallow seagrass beds APPENDIX B Figure B1. Figure B2. Figure B3. Figure B4. Figure B5. Figure B6. Figure B7. Figure B8. Figure B9. Figure B10. Figure B11. Figure B12. Figure B13. Scar-distribution map Escambia, Santa Rosa, Okaloosa counties Scar-distribution map Walton, Bay, Gulf counties Scar-distribution map Franklin, Wakulla, Jefferson counties Scar-distribution map Taylor, Dixie counties Scar-distribution map Levy, Citrus, Hernando counties Scar-distribution map Pasco, Pinellas, Hillsborough counties Scar-distribution map Manatee, Sarasota, Charlotte counties Scar-distribution map Lee, Collier counties Scar-distribution map Monroe County Scar-distribution map Dade, Broward counties Scar-distribution map Palm Beach, Martin counties Scar-distribution map St. Lucie, Indian River counties Scar-distribution map Brevard, Volusia counties iii

8 Acknowledgments This research was partially funded under a Coastal Zone Management grant pursuant to National Oceanic and Atmospheric Administration Award No. NA170Z0501. A number of people deserve special recognition for their contributions to this research: Chris Friel, Henry Norris, Jim Poehlman, and Bill Sargent in the Coastal and Marine Resources Assessment (CAMRA) group at the FMRI were substantially involved in the design, input and output, field-survey, and editing aspects of this project. Special thanks are also extended to Lt. Mike Wood, Florida Marine Patrol, for his patience during the aerial surveys and to Randolph L. Ferguson, National Marine Fisheries Service (NOAA), and Roy R. Lewis, III, Lewis Environmental Services, Inc., for manuscript review. We also thank the following people for their assistance: Everglades National Park: Skip Snow Florida Department of Environmental Protection, Division of Law Enforcement, Florida Marine Patrol: C. B. Goldacre, Fred Klohn, Lt. Steve Stout, and Lt. Paul Whitley Florida Department of Environmental Protection, Division of Marine Resources, Florida Marine Research Institute: Bredin Cummings, Sandra Drumm, Mike Durako, Ken Haddad, John Hunt, Chuck Idelberger, Gail MacAulay, Tom Matthews, Bob McMichael, Mike Mitchell, Jim Quinn, and Linda Tripodo Florida Department of Environmental Protection, Division of State Lands, Aquatic Preserves: Brian Poole, Kalani Cairns, Matthew Clemons, John Hughes, Judith Ott, Robert Repenning, Bradford Rieck, and Heather Stafford Monroe County: Virginia Barker and George Garrett South Florida Water Management District: Patti Sime and Les Vilcheck Southwest Florida Water Management District: Steve Dicks, Hugh Dinkler, Bob Evans, and Tom Lo St. Johns River Water Management District: Bob Virnstein Tampa Bay Regional Planning Council: Peter Clark University of South Florida, New College: Ruth Folit and Julie Morris iv

9 Scarring of Florida s Seagrasses: Assessment and Management Options Executive Summary Seagrasses are submerged, grass-like plants that inhabit the shallow coastal waters of Florida. Seagrasses are a vital component of Florida s coastal ecology and economy; they provide nutrition and shelter to animals important to marine fisheries, provide critical habitat for many other animals (e.g., wading birds, manatees, and sea turtles), and improve water quality. Marine-habitat degradation in Florida is continuing at an alarming rate as the coastal residential population and the number of seasonal visitors increase. Habitat degradation has many sources (e.g., pollution, dredge and fill), but an increasingly common cause of habitat degradation is the scarring of seagrasses. In this report, scarring can refer to either the activity of scarring or to a group of scars in a seagrass bed. Seagrass beds can be scarred by many activities, but scars are most commonly made when a boat s propeller tears and cuts up roots, stems, and leaves of seagrasses, producing a long, narrow furrow devoid of seagrasses. Boats operating in shallow waters are severely scarring, and sometimes completely denuding, seagrass beds throughout the state. The Florida Department of Environmental Protection recognized the need to reduce scarring of seagrasses by boats and committed resources to address this issue. As one component of this effort, the Florida Marine Research Institute (FMRI) investigated the distribution of scarred seagrass beds in the shallow marine waters of Florida s coastal counties. Aerial photography was used to locate seagrass scarring. Aerial surveys were then conducted in to confirm the location of scarred seagrasses. We did not attempt to distinguish among the different specific causes of seagrass scarring. During aerial surveys, observations of scarred seagrasses were recorded on National Oceanic and Atmospheric Administration nautical charts and U.S. Geological Survey quadrangle maps. Scarring intensity was categorized as light, moderate, or severe. Areas with substantial scarring recognizable on 1:24,000-scale photography were delineated on the maps with polygons, which were assigned a scarring intensity. Polygons categorized as light contained less than 5 percent scarring, those categorized as moderate contained 5 20 percent scarring, and those categorized as severe contained more than 20 percent scarring. The information acquired in this survey was incorporated into the FMRI s Marine Resources Geographic Information System (MRGIS), which produces maps and tabular products so that geographically based data can be effectively disseminated to resource managers, appropriate regional and county governments, and other interests (e.g., conservation groups and private citizens). Scarred seagrasses were observed in all areas of the state, mostly in shallow coastal waters less than six feet deep. More than 173,000 acres of the state s 2.7 million acres of seagrasses were scarred most of it lightly. This is a conservative estimate of scarring because we mapped groups of scars, not isolated, individual propeller scars. The total seagrass acreage in Florida (2.7 million acres) includes areas in the Florida Keys that have sparse seagrass and hardbottom with dense-seagrass patches. Excluding these areas, seagrasses totaled approximately 1.9 million acres. Also, these totals do not include sparse, deep Halophila beds that are offshore in the Big Bend region. The greatest acreage of moderate and severe (M/S) scarring occurred in areas having denser human populations and more registered boats. The Florida Keys (Monroe and Dade counties), Tampa Bay (Hillsborough, Manatee, and Pinellas counties), Charlotte Harbor (Lee County), and the north Indian River Lagoon (Brevard and Volusia counties) had the greatest M/S scarring. Monroe County, which includes most of the Florida Keys, had the greatest acreage of M/S scarring of all counties in the survey. The Panhandle and Big Bend regions had little M/S-scarred acreage, but in the western Panhandle embayments, M/S scarring was prevalent in the few acres of seagrasses there. If an area has little seagrass acreage, then any scarring may have a critical effect on habitat functions.

10 Scarring of Florida s Seagrasses F.J. Sargent et al All boating user-groups are responsible for scarring seagrasses. Although we did not attempt to identify each user-group s role in scarring, we believe general statements about the situations that lead to scarring are valid. The most severe single instances of scarring are caused by large commercial vessels, but most seagrass disruption results from widespread scarring by smaller boats. Our discussions with boaters, as well as our own personal experiences, suggest that scarring of seagrasses could result when one or more of the following situations occur: when boaters misjudge water depth and accidentally scar seagrass beds; when boaters who lack navigational charts or the skill to use them stray from poorly marked channels and accidentally scar seagrass beds; when boaters intentionally leave marked channels to take shortcuts through shallow seagrass beds, knowing that seagrass beds may be scarred; when boaters carelessly navigate in shallow seagrass beds because they believe scars heal quickly; when inexperienced boaters engage in recreational and commercial fishing over shallow seagrass flats, thinking that their boat s designed draft is not deep enough to scar seagrasses or that the design will prevent damage to their boat; when boaters overload their vessels, causing deeper drafts than the boaters realize; when boaters anchor over shallow seagrass beds, where their boats swing at anchor and scar seagrasses; when boaters intentionally prop-dredge to create a channel; and when inexperienced boaters, ignorant of what seagrasses are and the benefits they provide, accept as the behavioral norm local boating customs that disregard the environment. Management programs that address scarring of seagrasses should be based on an approach that involves (1) education, (2) channel marking, (3) increased enforcement, and (4) limited-motoring zones. Aerial monitoring and photography of the managed area are essential in evaluating the effectiveness of a program. Management programs that use this multifaceted approach have been instituted by a few local governments and at several state parks. Initial results of the programs indicate that in some areas seagrass scarring has been reduced but that in other areas emphasis may need to be increased on one or more of the components of the four-point approach. A statewide management plan is needed to address the most egregious scarring over large areas that may be difficult to regulate at the local-government level. Executive Summary FMRI Technical Report TR-1

11 Scarring of Florida s Seagrasses: Assessment and Management Options Introduction Seagrasses are completely submerged, grass-like plants that occur mostly in shallow marine and estuarine waters. Seagrasses form small, patchy beds if their seedlings have recently colonized bare sediments or if sediment movement or other disturbances disrupt typical growth patterns. Where disturbances are minimal and conditions promote rapid growth, large continuous beds known as meadows may develop when patchy seagrass beds coalesce. Seagrass meadows may require many decades to form. In shallower waters of good quality, seagrass meadows may be lush and have a high leaf density, but in deeper waters, they may be sparse, or species composition may shift to a less robust species. The predominant seagrass species in Florida (Figure 1) are turtle-grass (Thalassia testudinum Banks ex Koenig), shoal-grass (Halodule wrightii Aschers.), and manatee-grass (Syringodium filiforme Kutz.). Other, less common, seagrasses star-grass (Halophila engelmannii Aschers.), paddle-grass (Halophila decipiens Ost.), Johnson s seagrass (Halophila johnsonii Eisem.), and widgeon-grass (Ruppia maritima L.) may be locally abundant. Near river mouths subject to salinity fluctuations, other submerged aquatic plant species (e.g., Zannichellia sp. and Najas spp.) may occupy an ecologic role similar to that of the true marine seagrasses. Nevertheless, these species are rarely considered part of the seagrass flora of Florida. Turtle-grass is the largest of Florida s seagrass species, and Johnson s seagrass is the most diminutive. Johnson s seagrass was only recently recognized and named as a distinct species (Eiseman and McMillan 1980). Unlike the other species, which are widespread in Florida, Johnson s seagrass is limited to scattered locations in the lagoonal river systems of Florida s Atlantic coast. Because of its fragile nature, restricted distribution, and vulnerable status in the lagoonal systems (from development), Johnson s seagrass has been nominated for federal listing as a threatened species under the Endangered Species Act of The wide distribution and robust nature of turtle-grass belie its susceptibility to stress. Turtlegrass s tolerances, in respect to some environmental factors, are less developed than are those of some of the other seagrass species. Shoal-grass and widgeon-grass, for instance, are much more tolerant of periodic exposure during extremely low tides and consequently can flourish in shallower water than turtle-grass can. Manatee-grass has wiry leaves round in cross section that are more tolerant of strong currents. Like turtle-grass, manatee-grass is less tolerant of exposure and is often found mixed with turtle-grass at depths that are rarely exposed at extremely low tides. Species of Halophila are generally more tolerant of lower light conditions and usually form sparse beds in deeper waters, especially in the Gulf of Mexico offshore of Florida s Big Bend region. The numerous plants and animals that live and grow among seagrasses form a complex, fragile community. Marine and estuarine animals especially larval and juvenile fish benefit from seagrasses, which provide critical shelter and sustenance. Seagrasses form some of the most productive communities in the world (Zieman and Zieman 1989) and are aesthetically and economically valuable to humans. Seagrasses are a principal contributor to the marine food web and ultimately provide humankind with much of its seafood (Thayer et al. 1975). In addition, seagrasses improve water quality by stabilizing mobile sediments and by incorporating some pollutants into plant biomass and into the stabilized sediments. As Florida s population increases, particularly in coastal counties, threats to seagrass communities increase (Livingston 1987). Seagrass losses in Florida have been documented to range from 30 percent in the Indian River Lagoon (Haddad and Harris 1985) to 81 percent in Tampa Bay (Lewis et al. 1985). The cumulative effects of anthropogenic threats (e.g., water pollution, docks, dredging and filling) are being addressed by various federal, state, and local resource management programs. One threat that is becoming more acute as people increasingly use boats andother watercraft for recreation and work is scarring of seagrasses. In

12 Scarring of Florida s Seagrasses F.J. Sargent et al Thalassia testudinum Turtle-grass Halodule wrightii Shoal-grass Halophila johnsonii Johnson's Seagrass Halophila decipiens Paddle-grass Halophila engelmannii Star-grass Ruppia maritima Widgeon-grass Syringodium filiforme Manatee-grass Figure 1. Seagrass species occurring in the shallow coastal waters of Florida (based on drawings by Mark D. Moffler). 2 FMRI Technical Report TR-1

13 F.J. Sargent et al Scarring of Florida s Seagrasses this report, scarring can refer to either the activity of scarring or to a group of scars in a seagrass bed. Boat propellers scar seagrasses more often than do other sources. Most scarring of seagrasses is caused by smallboat propellers; however, larger craft, which are usually confined to deeper waters, may have much larger individual effects when they run aground, especially near shipping channels and ports. Propeller scarring of seagrasses was commented on in the scientific literature as early as the late 1950s (Woodburn et al. 1957, Phillips 1960). Concern has occasionally been voiced since then (e.g., U.S. Dept. of the Interior 1973, Chmura and Ross 1978). Eleuterius (1987) noted that scarring in Louisiana seagrasses was common and in deeper water was caused by shrimp boats, which also ripped up the margins of the beds with their trawls. Shrimper-related scarring and seagrass damage were also recognized by Woodburn et al. (1957). Usually, propeller scarring of seagrasses occurs when boaters motor through water that is shallower than the drafts of their boats. The propellers tear and cut up seagrass leaves, roots, stems, and sediments, creating unvegetated, lightcolored, narrow furrows called prop scars. In some areas, watercraft have extensively scarred seagrass beds, which has alarmed environmentally concerned citizens (Wilderness Society et al. 1990). In the Florida Keys, for example, as waterfront and recreational development has increased since the 1970s, so has the number, size, and power of vessels in this region resulting in widespread, and in some cases severe, scarring of shallow seagrass communities. Nearly all shallow seagrass beds in Florida show some degree of scarring. Portions of seagrass beds throughout the state have been completely denuded by repeated scarring (Figure 2). The degree of scarring depends, among other things, on the interaction between water depth and the size, kind, and speed of the boat. Vessels with more than one motor can have a much greater single-event effect on seagrasses than do singlemotored (and usually smaller) vessels. Several parallel tracks through a seagrass bed are a strong indicator that a multiple-motored vessel has probably passed that way. At lower tides, seagrass beds are more susceptible to scarring, even from a boat that would not scar them at higher tides. At high tide, a boat may navigate safely over seagrasses without scarring them, but at medium to low tide on the return trip, the same boat may scar them. A smaller boat operating at a slow speed or powering up may scar seagrass beds that would not be scarred after the boat reaches a plane. A boater s attitude is another factor that may influence the degree of scarring. Sometimes boaters are aware of but unconcerned about seagrasses and therefore do not avoid scarring them; A conscientious boater who trims his motor may only scar seagrasses slightly when he inadvertently enters a bed. A more extreme form of scarring occurs when a boater intentionally uses the boat s propeller as a dredge to remove seagrasses and sediments to produce a channel so that the boater can have easier access to other areas. This is called prop-dredging, and in some areas, it has permanently prohibited seagrass recovery, especially if sediments were dredged to bare rock. Currently, prop-dredging is illegal (see U.S.A. and FDER v. M.C.C. of Florida and Michael s Construction Company, Case No CIV-EBD, Southern District of Florida) but is difficult to enforce. Although everyday boating activities which may repeatedly scar seagrasses over extensive areas are more difficult to control because they are less overt, they may ultimately do greater harm to overall seagrass productivity than prop-dredging alone does. Substantial scarring of shallow seagrass beds, which are critical feeding and sheltering areas for wading birds, juvenile finfish, and shellfish, results in a cumulative reduction of productive habitat. Extensive scarring may expose the beds to further disruption from storms and other natural erosional forces, thereby increasing the rate of cumulative loss. This can result in the resuspension of sediments in the water column, which may further contribute to habitat loss by inhibiting the growth of seagrasses. Location and species composition of seagrass beds are probably principal determinants of the kind of animal habitat lost to scarring; however, comprehensive data do not exist concerning animal distributions in most seagrass areas of Florida. FMRI Technical Report TR-1 3

14 Scarring of Florida s Seagrasses F.J. Sargent et al Figure 2. Aerial view of seagrass-bed scarring at Lignumvitae Key in the Florida Keys. 4 FMRI Technical Report TR-1

15 F.J. Sargent et al Scarring of Florida s Seagrasses Seagrass Recovery Seagrass scarring has received limited study since the 1970s (e.g., Godcharles 1971, Zieman 1976), and only recently, as scarring has increased, has research become more focused on scar recovery (e.g., Matthews et al. 1991, Durako et al. 1992). Research on many aspects of seagrass biology and ecology has contributed greatly to our ability to protect marine resources. This research has shown that each of Florida s seagrass species has structural and physiological differences that affect their growth characteristics, stress tolerances, and ecologic contributions. As with other elements of seagrass ecology, scar recovery differs for each of the seagrass species. Seagrass species differ in their growth forms, particularly in the relationship between their rhizomes (underground stems) and leaves (Duarte et al. 1994). Some seagrass species rhizomes are weakly differentiated for vertical growth, and these plants may be more vulnerable to burial by mobile sediments. Other species (especially turtle-grass) have more strongly developed vertical rhizomes (i.e., short-shoots) and so can withstand some sedimentation (Figure 3). Because of this differentiation, branching and lateral growth are usually slower in species with the latter morphology, and scar recovery is also likely to be slower. Zieman (1976) attributed slow recovery of scars in turtle-grass beds to unsuitable sediment quality, damaged rhizomes, and the naturally slow growth of rhizome tips. He suggested that shoal-grass recovers more quickly than turtle-grass does because shoalgrass has a shallower rhizome system and grows well from seed. Also, shoal-grass probably recovers faster because its rhizomes have a greater density of short-shoots and nodes from which lateral branching occurs than does turtle-grass (Durako et al. 1992). DIFFERENTIATION OF VERTICAL RHIZOME Zostera Posidonia Halodule Syringodium Thalassia No vertical rhizomes Branches grow vertically or horizontally Short-shoots, but still with horizontal leaves Leaves only on short-shoots Figure 3. Gradient of horizontal and vertical rhizome differentiation in different seagrass genera, from those with no differentiation to those that only have leaves on the vertical rhizomes (after Duarte et al. 1994). Species within genera may differ in form. Gradient does not imply a phylogenetic relationship. FMRI Technical Report TR-1 5

16 Scarring of Florida s Seagrasses F.J. Sargent et al Studies in which trenches were cut 6 to 18 inches deep into seagrass beds indicated that seagrasses may recover slowly from scarring (Jones 1968 cited in Zieman 1976, Godcharles 1971). The trenches readily refilled with sediment, but seagrass regrowth was minimal even after two years. Zieman (1976) found that turtle-grass may require at least two years to begin to recolonize scars; even after five years, some scars were still visible. In a more recent study, Durako et al. (1992) used a linear-regression model to predict that individual scars 0.25 m wide cut into the centers of shoalgrass beds require years to recover to a normal density ( short-shoots/m 2 ) but that turtle-grass takes approximately years to achieve normal density ( shortshoots/m 2 ). At the sparser edges of shoal-grass beds, however, recovery times ( years) approached those of turtle-grass, probably because of lower nodal densities at the margins of shoalgrass beds. Some researchers have indicated that complete seagrass-scar recovery may take as long as ten years, depending on the size of the denuded area (Lewis and Estevez 1988). For seagrass to recover in the shortest period, scarred areas must remain free of additional scarring, and other environmental conditions must be favorable for plant survival and growth. Even so, the recovery period for scarred seagrass beds (especially for turtlegrass) averages at least three to five years and is probably much longer in areas of poor water quality and where scarring is severe and repetitive. Some scarred seagrass beds may never recover. The rate of seagrass recovery from scarring depends on many factors. Some of the variables that may affect recovery from scarring are sediment composition, water quality, current velocity, wave and wind energy, drift algae, scar depth, seagrass species, water depth, and latitude. Sediment properties and water quality are overriding determinants of recovery from scarring. Seagrasses absorb nutrients from the sediments in which they are rooted and also derive nutrition from the water column. Durako et al. (1992) suggested that south Florida sediments, which are usually carbonaceous marl muds, could affect seagrass regrowth differently than do the predominantly quartz-sand sediments of Tampa Bay. Over short distances, sediment quality may vary significantly; sediments in scars can differ in quality even from adjacent, undisturbed seagrass sediments. In the Florida Keys, for example, soil particle sizes were coarser in scars, and sediments had a lower ph and EH (Zieman 1976). In Tampa Bay, by contrast, particle-size distributions did not differ between scars and adjacent seagrass sediments (Dawes et al. 1994). Therefore, sediment type and other local conditions may affect whether scar sediments differentiate from adjacent unscarred seagrass sediments. Water quality (e.g., salinity and clarity) affects plant physicochemical attributes such as osmotic balance and photosynthetic rates and, therefore, it can affect the amount of energy available for seagrass growth. Some seagrass species tolerate much lower salinities than others do. Turtle-grass, for example, does not survive for long in salinities below parts per thousand (Lewis et al. 1985, Dawes 1987). Although Eleuterius (1987) observed that widgeon-grass could withstand totally fresh water, he found that of the truly marine seagrasses, shoal-grass was the most tolerant of low salinities and star-grass was the least tolerant. Turtlegrass and manatee-grass were intermediate in their responses to lower salinity. In areas where frequent and large freshwater pulses are common (e.g., near the mouths of rivers), recovery rates will be faster in seagrass species that tolerate lower salinities (i.e., shoal-grass and widgeon-grass). Shading experiments and surveys of seagrass extents in turbid waters have shown that light reduction lowers shoot density and reduces survivability (Hall et al. 1991, Onuf 1991). Sediments that are composed mainly of finer particle sizes are more subject to resuspension (Gucinski 1982) and could pose a threat to photosynthetic processes in seagrasses. Sediment resuspension and water clarity are affected by current velocity, wave and wind energy, and nutrient fluxes, among other things. In particular, drift algae may respond vigorously to higher nutrient levels and depress scarring-recovery rates by physically inhibiting seagrass growth (e.g., Holmquist 1992) and photosynthesis and by accumulating in scars. Water depth influences photosynthetic rates and seagrass growth, especially in nutrient-rich waters. Seagrasses in deeper water receive lower amounts of solar radiation and a different quality of light, both of which could affect energy-allocation patterns. Energy-allocation patterns of seagrasses can also be affected by latitude. Latitude, coupled with other local environmental variables, 6 FMRI Technical Report TR-1

17 F.J. Sargent et al Scarring of Florida s Seagrasses affects seagrass growth because of differing watertemperature, solar-incidence, and day-length regimes. The warmer water, longer day-lengths, and more intense solar radiation occurring at lower latitudes probably enhance seagrass growth rates and fruit production in deeper or more turbid water. Therefore, potential recovery and recolonization rates may be faster for seagrasses in the Florida Keys than in the Florida Panhandle. However, local physicochemical conditions, such as sediment characters, may override latitudinal effects. Scar depth probably affects regrowth rate as well. Deeper scars may not fill with sediment, or may become enlarged, if they occur in areas of strong currents (Zieman 1976, Eleuterius 1987). Scars in shallow-water seagrass beds that are exposed to long wind fetches may be scoured by strong winds and waves, especially during extremely low tides. Boat wakes can also scour scarred areas. Kenworthy et al. (1988) concluded that boat-wake waves substantially elevate bottom-shear stress along shallow seagrass beds and seriously jeopardize seagrass health. Study Objectives Slow recovery from scarring, coupled with increased scarring rates, elevates the rate of cumulative loss of seagrasses and their habitat values. Concerns about the effects of seagrass scarring and recovery on marine productivity compelled the Florida Marine Research Institute (FMRI) to survey the extent and intensity of scarring in the shallow coastal waters of Florida. Information from this general study is intended to assist government agencies with developing specific management programs in regard to boat-generated scarring of seagrasses. A general survey of the extent and intensity of scarring is the necessary first step in developing appropriate and cost-effective management protocols. This report identifies and quantifies the extent of scarred seagrass beds throughout most of Florida. We collected and analyzed the data using a combination of aerial photography, aerial surveys, and Geographic Information System (GIS) technology. For the first time, the statewide extent of seagrasses is described, and the magnitude of scarring is estimated and documented so that Florida s seagrass resources can be more effectively protected. Based on the data and anecdotal observations generated in this study, we identify and discuss behavioral activities and navigational circumstances that exacerbate seagrass scarring. Further investigations and surveys using developing technologies will refine our knowledge of seagrass distributions and the effects of human activities on the resource s productivity. Methods The main goal of this project was to survey Florida s shallow marine and estuarine waters for scarring of seagrasses. For most of Florida, we used an approach combining analysis of high-resolution aerial photographs with ground-truthing during aerial surveys. In the Florida Keys, the aerial surveys were conducted first, and aerial photography was used as collateral data. The study area extended from the Alabama- Florida border at Perdido Bay (Escambia County), east and south along the Gulf coast to the Florida Keys, and then north along the lagoonal river systems of the Atlantic Coast to just south of New Smyrna Beach (Volusia County) in Mosquito Lagoon. A total of 31 of the state s 35 coastal counties are included in this survey (Figure 4). The four counties north of Volusia County on the Atlantic coast of Florida were not included because areas suitable for seagrass growth are not present. Only the southern part of Volusia County below U.S. Highway A1A at Port Orange was included in this survey. Even though seagrass scars can result from many sources (e.g., ship groundings, live-aboard houseboats, and even four-wheel-drive vehicles), boat propellers are the most widespread cause of scarring. In this study, we did not distinguish among the various scarring sources. Individual prop-scar widths are narrow and average approximately 12 inches; scar lengths vary considerably, from miles to only yards long and can be difficult to see in aerial photographs. In a previous study of scar recovery, Durako et al. (1992) suggested that the smallest-scale (least detailed) aerial photography useful for recognizing scars in seagrass beds is 1:24,000 (1 inch = 2,000 ft). A greater number of scars can be identified using larger-scale photography (e.g., 1:2,400). At a single site in Tampa Bay, Durako et al. (1992) were able to distinguish 700 individual scars in 1:2,400-scale photography, 104 FMRI Technical Report TR-1 7

18 Scarring of Florida s Seagrasses F.J. Sargent et al Alabama Georgia Santa Rosa Escambia Okaloosa Walton Bay Gulf Jefferson Wakulla Taylor Franklin Atlantic Ocean Dixie Levy Citrus Volusia Hernando Pasco Hillsborough Pinellas Brevard Indian River Gulf Manatee St. Lucie of Mexico Sarasota Charlotte Lee Martin Palm Beach Collier Broward Dade Monroe N W E S Figure 4. Study area for assessment of seagrass scarring. 8 FMRI Technical Report TR-1

19 F.J. Sargent et al Scarring of Florida s Seagrasses scars at 1:12,000, and only 5 scars with 1:24,000. Nevertheless, they suggested that most of the heavily scarred areas could be identified at the 1:24,000 scale and that the trade-off in the time saved using 1:24,000 photography justified its use. Even though aerial photography can provide sufficient detail to allow recognition of prop scars, high-detail photography is often limited to certain areas. Pertinent photography not contained in the FMRI library was obtained from the appropriate water-management districts. The largest-scale aerial photography available was 1:13,200 colorinfrared (CIR) transparencies made for the Florida Keys Land Cover Mapping Project (funded by the U.S. Environmental Protection Agency s Advanced Identification of Wetlands Program) in December The smallest-scale photography used to delineate scarring in our study consisted of 1:40,000 CIR transparencies provided by the South Florida Water Management District (SFWMD). These photographs covered Hobe Sound, southeast Florida, Biscayne Bay, the upper Florida Keys and Florida Bay, and the southwest region of Florida from Florida Bay to Charlotte Harbor. The Southwest Florida Water Management District (SWFWMD) supplied 1:24,000 CIR photographs from Yankeetown south to Charlotte Harbor. The St. Johns River Water Management District (SJR- WMD) furnished 1:24,000 CIR photographs for Mosquito Lagoon and Indian River Lagoon. The only aerial photographs available for the Panhandle and Big Bend regions were approximately ten years old and therefore were too dated for delineating seagrass scarring for this study. The oldest photographs used for scarring delineation were taken in November Although these photographs did not represent conditions at the time of the survey, historical scarring patterns were documented from them, and areas requiring closer examination were identified. Some problems are inherent in using photography of different scales. In particular, comparing maps of different scales should be done with caution. Large-scale photography (e.g., 1:2,400) can give more accurate representations of seagrass distributions and scarring than smaller-scale (e.g., 1:40,000) photography can. Just the width of a line drawn on a small-scale photograph may contain many hundreds or thousands of acres of seagrass, depending on the line s length and the scale of the photography. One problem in implementing this study was that large-scale photography or even photography of the same scale for different areas of the state did not exist. Also, offshore county lines were based upon 1:100,000 TIGER cultural data, and subtle differences in county-line boundaries could alter conclusions if the data are used too strictly in detailed comparisons. Therefore, we urge caution when making comparisons of the differences between regions and between counties. Scarring Recognition Scarring was recognized as distinct areas of lightcolored lines and patches visible in photographs and from the air that contrasted with the darker colors of seagrass beds. Scarred areas in the 9 inch x 9 inch CIR aerial photographs were delineated using binocular macroscopes and stereoscopes, and the delineations were transferred to registered acetate overlays. Where scars merged, a bounding polygon was drawn around the entire scarred area (Figure 5). Polygons were only drawn around groups of scars, not around single, isolated prop scars. We did not map areas less than one acre due to the small-scale maps used. Because of the mapping procedure and differing map scales used, we may have inadvertently included small portions of bare substrate, channels, and open water in some polygons. For example, in areas that contained intricate shorelines with numerous islands such as the Ten Thousand Islands and the Chassa-howitzka and Crystal rivers delineating small polygons was impossible at the available map scales; as a consequence, some unscarred areas were incorporated within the polygons. The intensity of scarring in each polygon was categorized based upon the Comparison Chart for Visual Estimation of Percentage Composition (after Terry and Chilingar 1955). Polygons designated as light enclosed areas where less than 5 percent of the seagrasses were scarred, moderate polygons contained areas with from 5 percent to 20 percent scarring, and severe polygons delineated areas with more than 20 percent scarring. For example, a 20-acre polygon that was classified as being moderately scarred would contain between 1 and 4 acres of actual scars. Diagrammatic representations of the three categories of estimated scarring intensity are shown in Figure 6. In some areas, different intensities of scarring were adjacent and could not be easily differentiated. These areas were delineated as a single polygon and were assigned FMRI Technical Report TR-1 9

20 Scarring of Florida s Seagrasses F.J. Sargent et al Severe-Scarring Polygons Figure 5. Example of polygon delineation. a value for the overall scarring intensity. An oblique aerial photo in Figure 7 illustrates this situation. Information about seagrass scarring in Florida Bay was furnished by Skip Snow of the Everglades National Park (ENP). Within Florida Bay, scarring occurs principally on seagrass banks, which are exposed at low tide. To confirm the locations of scarred seagrasses, a brief aerial survey was conducted by FMRI staff over a portion of Florida Bay. Polygons drawn on the registered overlays on the aerial photographs were transferred to National Oceanic and Atmospheric Administration (NOAA) nautical charts using a zoom transfer scope (ZTS). The ZTS superimposes an image onto a base map of a different scale, providing for accurate transfer of the hand-drawn polygons from the photograph overlays onto the NOAA base maps. In most cases, 1:40,000-scale NOAA charts were used as base maps. The lack of larger-scale charts for the region from Anclote Key (Pasco County) to Alligator Harbor (Franklin County) forced us to use 1:80,000-scale charts. When possible, we used inset maps of various scales (1:5,000 1:20,000) to supplement small-scale chart information. In a portion of the Florida Keys, 1:24,000-scale U. S. Geological Survey (USGS) quadrangle maps were used as base maps because the largest-scale NOAA charts were only available at a scale of 1:80,000 (Table 1). Scar Mapping After marking the maps and charts with polygons, we conducted aerial surveys to verify scarring and refine the delineations of scarring intensity. Most aerial surveys were conducted between May 1992 and May The Florida Keys surveys were conducted between October 1992 and March Aerial surveys were important in assuring accurate representations of the extent and intensity of scarring because even in the better photographs, not all scars were visible. During the aerial surveys, boats were frequently observed scarring shallow seagrass beds. Some of these events were photographed, and the photographs were deposited in the FMRI library. 10 FMRI Technical Report TR-1

21 F.J. Sargent et al Scarring of Florida s Seagrasses Light Scarring Moderate Scarring Severe Scarring Figure 6. Diagrammatic representation of the three categories of estimated scarring intensity. Black space within each block represents seagrasses, and white marks represent scarring. Light scarring is defined as the presence of scars in less than 5 percent of the delineated polygon, moderate scarring as the presence of scars in 5 to 20 percent of the polygon, and severe scarring as the presence of scars in more than 20 percent of the polygon. The Indian River Lagoon, the southeast Intracoastal Waterway, and the Florida Keys were surveyed from light, fixed-wing aircraft (Cessna 152 or 172) in regions where seagrasses were distributed along relatively straight and continuous shorelines. Regions with convoluted shorelines and numerous islands, such as Tampa Bay, Biscayne Bay, Waccasassa Bay, and parts of Florida Bay, were surveyed from a helicopter (Hughes 500). In the lower Florida Keys, where wide areas of seagrass extend from the Atlantic Ocean into Florida Bay, transects approximately 1000 feet apart were conducted perpendicular to the main axis of the Florida Keys. The Intracoastal Waterway formed the boundary between Everglades National Park and the Florida Keys in this assessment. Military bases prohibited aerial surveys of some seagrass areas. FMRI Technical Report TR-1 11

22 Scarring of Florida s Seagrasses F.J. Sargent et al Severe Moderate Figure 7. Recognition of scarring intensity. Contiguous small polygons of different scarring intensities were combined into one overall intensity category. This seagrass bed would be recognized as severely scarred overall, even though part of it is only moderately scarred. Altitudes between 300 and 500 feet provided the best perspective for this study. Flight speeds were between 80 and 100 knots, depending on scar complexity and water clarity. Clear skies, calm seas, a vertical sun angle, and clear water were essential for conducting accurate aerial surveys. Rain and high winds made it difficult to see scars through the surface of the water. Glare from sunlight reflecting off the water in late afternoon and early morning also hampered observations. Turbidity caused by rough water during storms usually persisted for several days. Dark-colored, organically stained water discharged from rivers during and after rain storms greatly impeded our ability to identify scarred seagrasses during aerial surveys. After completing the aerial surveys, we edited and recompiled the scarring data onto a clean set of base maps and then transferred the data into the Marine Resources Geographic Information System (MRGIS) at the FMRI. Complete descriptions of the MRGIS integration process, statewide map-creation techniques, and error-correction methodology are in Appendix A. ARC/INFO software (v ) was used in this study to analyze scarring data and to produce output maps. Scarring information from this study is digitally stored and can easily be shared with other groups. All original base maps and photograph overlays have been archived at the FMRI. Results Moderately dense to dense seagrasses i.e., excluding sparse and hardbottom seagrasses in the Florida Keys and sparse Halophila beds elsewhere total approximately 1,901,000 acres. If hardbottom and sparse seagrasses in the Florida Keys are included in acreage estimates, seagrasses in Florida total nearly 2,660,000 acres (Table 2). The distribution of seagrasses in Florida coastal waters 12 FMRI Technical Report TR-1

23 F.J. Sargent et al Scarring of Florida s Seagrasses Table 1. NOAA nautical charts and USGS topographic maps used as base maps on which seagrass scarring in Florida was represented. Number Scale Official Name Chart :40,000 Intracoastal Waterway - Santa Rosa Sound to Dauphin Island Chart :40,000 Intracoastal Waterway - Lake Wimico to East Bay Chart :40,000 Intracoastal Waterway - Apalachicola to Lake Wimico Chart :40,000 Intracoastal Waterway - Carrabelle to Apalachicola Bay Chart :80,000 Apalachee Bay Chart :80,000 Horseshoe Point to Rock Islands Chart :80,000 Crystal River to Horseshoe Point Chart :80,000 Anclote Keys to Crystal River Chart :80,000 Tampa Bay and St. Joseph Sound Chart :40,000 Tampa Bay - northern part Chart :40,000 Tampa Bay - southern part Chart :40,000 Intracoastal Waterway - Charlotte Harbor to Tampa Bay Chart :40,000 Intracoastal Waterway - Fort Myers to Charlotte Harbor Chart :40,000 Everglades National Park - Lostmans River to Wiggins Pass Chart :50,000 Everglades National Park - Shark River to Lostmans River Chart :50,000 Everglades National Park - Whitewater Bay Chart :30,000 Key West Harbor and approaches Chart :80,000 Sombrero Key to Sand Key Chart :40,000 Intracoastal Waterway - Bahia Honda to Key West Chart :40,000 Intracoastal Waterway - Big Spanish Channel to Johnson Key Chart :40,000 Matecumbe to Bahia Honda Key Chart :80,000 Miami to Marathon and Florida Bay Chart :40,000 Intracoastal Waterway - Elliott Key to Matecumbe Chart :40,000 Intracoastal Waterway - Miami to Elliott Key Chart :40,000 Intracoastal Waterway - West Palm Beach to Miami Chart :40,000 Intracoastal Waterway - Tolomato River to Palm Shores USGS map 1:24,000 Marquesas Keys West USGS map 1:24,000 Marquesas Keys East USGS map 1:24,000 Cottrell Key is uneven; some counties have very little and others have a disproportionately large amount (see figures in Appendix B). Monroe County alone contains 54.6 percent of all Florida seagrass-bed acreage mostly in Florida Bay and the Florida Keys (Tables 2 and 3). Much of the remaining seagrass acreage (26.4 percent) occurs in the shallow Gulf waters of Taylor, Citrus, Hernando, Levy, and Dixie counties in the Big Bend region of Florida. These counties have more seagrasses because they have extensive, shallow-water, low-energy areas with water quality that is generally good. These conditions promote rapid growth and coalescence of seagrasses. Other extensive seagrass components in deeper waters in this area are species of Halophila, which are usually in sparse or patchy beds. We did not include these seagrass types in this survey. The remaining seagrass acreage (19 percent) is fairly evenly distributed among the other 25 coun- FMRI Technical Report TR-1 13

24 Scarring of Florida s Seagrasses F.J. Sargent et al Table 2. Acreage of scarred seagrasses (to nearest ten acres) in each Florida coastal county in this study. Totals in scarring categories are based on calculated values, not on rounded values. Light scarring is defined as the presence of scars in less than 5 percent of the delineated polygon, moderate scarring as the presence of scars in 5 to 20 percent of the polygon, and severe scarring as the presence of scars in more than 20 percent of the polygon. County Total Light Moderate Severe Moderate Total Seagrass Scarring Scarring Scarring +Severe Scarring BAY 10,530 4, ,950 BREVARD 46,190 4,160 1, ,050 6,210 BROWARD 0 (1) CHARLOTTE 14,190 1,530 5, ,910 7,440 CITRUS 147,810 25,700 1, ,880 27,580 COLLIER 5,250 1,970 1, ,680 3,650 DADE 145,650* 2,740 3,970 4,500 8,480 11,220 DIXIE 111,130 2,470 1, ,020 3,490 ESCAMBIA 2, FRANKLIN 19, GULF 8,170 4, ,840 HERNANDO 146,870 7, ,500 HILLSBOROUGH 6,320 1,680 2, ,410 4,090 INDIAN RIVER 2, JEFFERSON 10, LEE 50,510 5,930 7,100 1,290 8,390 14,310 LEVY 132,400 9, ,090 MANATEE 12,160 2,480 2, ,990 5,470 MARTIN 2, MONROE 1,452,800* 14,560 10,430 5,060 15,490 30,050 OKALOOSA 3, (5) PALM BEACH 2, PASCO 85,570 2,120 1, ,120 4,240 PINELLAS 22,920 3,800 3,870 2,010 5,880 9,680 SANTA ROSA 2, SARASOTA 4, ,050 ST. LUCIE 6, TAYLOR 162,860 8, ,160 VOLUSIA 8,490 1, ,370 2,800 WAKULLA 29,630 2, ,790 WALTON TOTAL 2,658,290* 109,870 48,630 15,470 64, ,960 * Dade County and Monroe County totals include sparse-seagrass areas and hardbottom areas that have dense patches of turtlegrass and shoal-grass intermixed. See Table 6 for a breakdown of seagrass acreage in these counties and the text for an explanation. The total area of moderately dense, dense, and contiguous seagrasses for the state is 1,900,960 acres, excluding hardbottom and sparse seagrasses in the Florida Keys and sparse Halophila in the Big Bend region. 14 FMRI Technical Report TR-1

25 F.J. Sargent et al Scarring of Florida s Seagrasses Table 3. Relative percentages of scarred seagrasses, by intensity level, in each Florida coastal county in this study. Relative percentage is calculated for each category as the scarring in the county divided by scarring for the state multiplied by 100. Light scarring is defined as the presence of scars in less than 5 percent of the delineated polygon, moderate scarring as the presence of scars in 5 to 20 percent of the polygon, and severe scarring as the presence of scars in more than 20 percent of the polygon. County Total Light Moderate Severe Moderate Total Seagrass Scarring Scarring Scarring +Severe Scarring BAY BREVARD BROWARD CHARLOTTE CITRUS COLLIER DADE DIXIE ESCAMBIA FRANKLIN GULF HERNANDO HILLSBOROUGH INDIAN RIVER JEFFERSON LEE LEVY MANATEE MARTIN MONROE OKALOOSA PALM BEACH PASCO PINELLAS SANTA ROSA SARASOTA ST. LUCIE TAYLOR VOLUSIA WAKULLA WALTON FMRI Technical Report TR-1 15

26 Scarring of Florida s Seagrasses F.J. Sargent et al ties, mostly in embayments and lagoonal systems. Twenty-two counties have less than 50,000 acres of seagrass, and the majority of those have less than 20,000 acres. The median seagrass acreage for the 31 coastal counties in this study is approximately 10,500 acres. After Monroe County (1,452,800 acres), Taylor county has the largest seagrass acreage (162,860 acres). Of the Florida counties that contain at least some seagrass, Broward County had the smallest acreage; approximately one acre of seagrass could be recognized from seagrass-distribution sources. The least amount of total scarring (the sum of the light, moderate, and severe categories) occurred in those counties that have little seagrass acreage (e.g., Broward, Indian River, and Walton). For scarring to be extensive, the first requirement is that a county must contain a substantial acreage of seagrass. Counties with little seagrass acreage, but with all of it scarred, would rank high in statewide scarring (Table 4). Therefore, ranking counties based on the percentage of seagrass scarred within the county can be deceptive. For comparative purposes, then, counties must be ranked based on their percentages of scarring relative to scarring for the entire state. Relative to the whole state, the greatest amount of total scarring occurred, as would be expected from seagrass distributions, in Monroe and Citrus counties (Tables 2, 3, and 5). Lee, Dade, Levy, and Pinellas counties also had substantial scarring. Of greatest immediate concern is scarring in the moderate and severe categories (M/S scarring). Scarring in the light category in most areas is probably not of immediate concern in protecting seagrasses, unless the area is subject to increasing boat use. The counties with the most M/S scarring were Monroe, Dade, Lee, Charlotte, and Pinellas. Most scarring in Citrus and Levy counties was in the light category, so these two counties are of lower importance when only M/S scarring is considered. Fourteen counties each had less than one percent of the state s M/S scarring. Of these, Walton, Broward, Martin, Palm Beach, St. Lucie, and Indian River counties had the lowest amounts of M/S scarring because they all have low seagrass acreage. Of the counties containing substantial acreages of seagrass (i.e., those with more than one percent of statewide coverage), Taylor, Hernando, Wakulla, Dixie, and Citrus counties had the least M/S-scarring acreage. These counties are all in the Big Bend region of Florida, which is sparsely populated and has low numbers of registered boats. These five counties account for 22.5 percent of the state s seagrass acreage. Scarring extents and intensities for all 31 coastal counties in this study are illustrated in the figures in Appendix B. Generalized seagrass distributions compiled from various sources may be misleading if data were based on different definitions for sparse seagrass or included patchy (but dense) seagrasses within a polygon. In this study, sparse and hardbottom seagrasses in Monroe and Dade counties were included in the overall seagrass distributions because substantial patches of dense and moderately dense shoal-grass and turtle-grass were intermixed and could not be separately delineated. In areas of the Big Bend and Indian River Lagoon, however, we deleted sparse-seagrass categories from mapping and analysis because they were mostly very sparse Halophila beds, which are usually in deeper waters and which may not be pertinent to ecological concerns addressed in this study. Nevertheless, we separated the seagrass distributions for Monroe and Dade counties into sparse/hardbottom and dense/moderately dense seagrass acreages (Table 6) for those who wish to eliminate these categories from scarring-extent calculations. All of our calculations were based on the total seagrass acreages for Monroe and Dade counties. Polygons representing scarring in areas where sparse seagrasses had been excluded from the generalized distribution were retained in the analysis because they indicated the presence of seagrasses, as confirmed in the aerial surveys. Caution must be used when assessing the meaning of the data presented in this study. Although we have attempted to reduce distribution errors, inaccuracies remain because of the broad nature of this type of study. Mapping of seagrasses and scarring will be in constant flux as more detailed data are generated for different areas. To more broadly identify differences in seagrass-scarring distribution, five regions (Figure 8) 16 FMRI Technical Report TR-1

27 F.J. Sargent et al Scarring of Florida s Seagrasses Table 4. Percentages of scarred seagrasses, by intensity level, within each Florida coastal county in this study. Light scarring is defined as the presence of scars in less than 5 percent of the delineated polygon, moderate scarring as the presence of scars in 5 to 20 percent of the polygon, and severe scarring as the presence of scars in more than 20 percent of the polygon. The percentage of scarred seagrasses for the entire state in each category is light = 4.1%, moderate = 1.8%, severe = 0.6%, moderate + severe = 2.4%, and total scarring = 6.5%. Total Percent Percent Percent Percent Percent County Seagrass Light Moderate Severe Moderate Total Acres Scarring Scarring Scarring +Severe Scarring BAY 10, BREVARD 46, BROWARD CHARLOTTE 14, CITRUS 147, COLLIER 5, DADE 145, DIXIE 111, ESCAMBIA 2, FRANKLIN 19, GULF 8, HERNANDO 146, HILLSBOROUGH 6, INDIAN RIVER 2, JEFFERSON 10, LEE 50, LEVY 132, MANATEE 12, MARTIN 2, MONROE 1,452, OKALOOSA 3, PALM BEACH 2, PASCO 85, PINELLAS 22, SANTA ROSA 2, SARASOTA 4, ST. LUCIE 6, TAYLOR 162, VOLUSIA 8, WAKULLA 29, WALTON FMRI Technical Report TR-1 17

28 Scarring of Florida s Seagrasses F.J. Sargent et al Table 5. County rankings of scarred-seagrass acreage, by scarring intensity, in each Florida coastal county in this study. Rank is in decreasing order of acreage scarred. Counties with the same acreage are ranked alphabetically. Light scarring is defined as the presence of scars in less than 5 percent of the delineated polygon, moderate scarring as the presence of scars in 5 to 20 percent of the polygon, and severe scarring as the presence of scars in more than 20 percent of the polygon. Total Light Moderate Severe Moderate Total Seagrass Scarring Scarring Scarring +Severe Scarring 1 MONROE CITRUS MONROE MONROE MONROE MONROE 2 TAYLOR MONROE LEE DADE DADE CITRUS 3 CITRUS LEVY CHARLOTTE PINELLAS LEE LEE 4 HERNANDO TAYLOR DADE LEE CHARLOTTE DADE 5 DADE HERNANDO PINELLAS MANATEE PINELLAS LEVY 6 LEVY LEE HILLSBOROUGH PASCO MANATEE PINELLAS 7 DIXIE GULF MANATEE VOLUSIA HILLSBOROUGH HERNANDO 8 PASCO BREVARD BREVARD CHARLOTTE PASCO TAYLOR 9 LEE BAY PASCO CITRUS BREVARD CHARLOTTE 10 BREVARD PINELLAS CITRUS HILLSBOROUGH CITRUS BREVARD 11 WAKULLA DADE COLLIER BREVARD* COLLIER MANATEE 12 PINELLAS* MANATEE DIXIE GULF* VOLUSIA BAY 13 FRANKLIN* DIXIE VOLUSIA COLLIER* DIXIE GULF 14 CHARLOTTE* PASCO BAY BAY* BAY PASCO 15 MANATEE* WAKULLA WAKULLA INDIAN RIVER* WAKULLA HILLSBOROUGH 16 BAY* COLLIER HERNANDO SARASOTA* HERNANDO COLLIER 17 JEFFERSON* HILLSBOROUGH GULF ESCAMBIA* GULF DIXIE 18 GULF* CHARLOTTE FRANKLIN* BROWARD* FRANKLIN* VOLUSIA 19 VOLUSIA* VOLUSIA SARASOTA* DIXIE* SARASOTA* WAKULLA 20 ST. LUCIE* SARASOTA* ESCAMBIA* FRANKLIN* ESCAMBIA* SARASOTA* 21 HILLSBOROUGH* ESCAMBIA* LEVY* HERNANDO* LEVY* FRANKLIN* 22 COLLIER* SANTA ROSA* SANTA ROSA* JEFFERSON* SANTA ROSA* ESCAMBIA* 23 SARASOTA* FRANKLIN* JEFFERSON* LEVY* JEFFERSON* SANTA ROSA* 24 OKALOOSA* JEFFERSON* OKALOOSA* MARTIN* OKALOOSA* JEFFERSON* 25 INDIAN RIVER* OKALOOSA* TAYLOR* OKALOOSA* TAYLOR* OKALOOSA* 26 ESCAMBIA* INDIAN RIVER* ST. LUCIE* PALM BEACH* INDIAN RIVER* INDIAN RIVER* 27 SANTA ROSA* PALM BEACH* PALM BEACH* SANTA ROSA* ST. LUCIE* ST. LUCIE* 28 PALM BEACH* ST. LUCIE* INDIAN RIVER* ST. LUCIE* PALM BEACH* PALM BEACH* 29 MARTIN* MARTIN* MARTIN* TAYLOR* MARTIN* MARTIN* 30 WALTON* WALTON* BROWARD* WAKULLA* BROWARD* WALTON* 31 BROWARD* BROWARD* WALTON* WALTON* WALTON* BROWARD* * Relative percentage is less than one percent. 18 FMRI Technical Report TR-1

29 F.J. Sargent et al Scarring of Florida s Seagrasses Table 6. Acreages (to nearest ten acres) of seagrass-density categories in the Florida Keys. County Total Seagrass Moderate/Dense Seagrass Sparse/Hardbottom Seagrass DADE 145, ,680 24,320 MONROE 1,452, , ,210 TOTAL 1,598, , ,530 were demarcated in the analysis of scarring extents and intensities: Region 1. Panhandle (Escambia County Franklin County), Region 2. Big Bend (Wakulla County Pasco County), Region 3. Gulf Peninsula (Pinellas County Lee County), Region 4. Atlantic Peninsula (Palm Beach County Volusia County), and Region 5. South Florida (Collier County Broward County). Acreages of scarred seagrasses occurring in these regions are in Table 7. The areas of Florida with the greatest acreages of M/S scarring were the Gulf Peninsula and South Florida regions. Based only on the severe-scarring category, however, the South Florida region had twice the scarred acreage of the Gulf Peninsula region. If the light-scarring category is included, the Big Bend region had the greatest total of scarred-seagrass acreage. However, the light-scarring category may not be of greatest concern in protecting seagrasses from scarring; therefore, the Big Bend region may not be a priority for a management program, except for protecting sites where M/S scarring occurs and ensuring that scarring does not become worse. When M/S scarring is viewed relative to the total seagrass acreage in the region, the most threatened region is the Gulf Peninsula (23.5 percent of its seagrasses scarred); it has extensive scarring relative to the moderate acreage of seagrasses there. Because of the extensive acreages of seagrasses in the South Florida and Big Bend regions, scarring levels (1.6 percent and 0.8 percent of their seagrasses scarred) were low relative to the area of total seagrasses present. However, most of these seagrasses occur in water depths where they are unlikely to be scarred. Region 1. Panhandle: This region has the least acreage of seagrass in the state (Table 7). Bay and Table 7. Acreages of scarred seagrasses (to nearest ten acres) in each region of Florida demarcated in this study. Light scarring is defined as the presence of scars in less than 5 percent of the delineated polygon, moderate scarring as the presence of scars in 5 to 20 percent of the polygon, and severe scarring as the presence of scars in more than 20 percent of the polygon. Region Total Light Moderate Severe Moderate Total Seagrass Scarring Scarring Scarring +Severe Scarring 1. PANHANDLE 48,170 9,970 2, ,290 12, BIG BEND 826,770 58,630 6, ,720 65, GULF PENINSULA 110,260 16,140 21,330 4,580 25,910 42, ATLANTIC PENINSULA 69, , ,520 3, SOUTH FLORIDA 1,603,700* 19,270 15,990 9,650 25,640 44,910 * South Florida total includes sparse-seagrass areas and hardbottom areas with moderately dense and dense patches of turtlegrass and shoal-grass intermixed. See Table 6 for a breakdown of seagrass acreage in these counties and the text for an explanation. The total area of moderately dense, dense, and contiguous seagrasses in the state is 1,900,960 acres, excluding hardbottom and sparse seagrasses in the Florida Keys and sparse Halophila beds in the Big Bend region. FMRI Technical Report TR-1 19

30 Scarring of Florida s Seagrasses F.J. Sargent et al Alabama Georgia Santa Rosa Escambia Okaloosa Walton Bay Gulf Franklin Wakulla Jefferson Taylor Atlantic Ocean Dixie Levy Citrus Volusia Hernando Pasco Hillsborough Pinellas Brevard Indian River Gulf of Manatee Sarasota Charlotte St. Lucie Martin Mexico Lee Palm Beach Collier Broward Dade Monroe Region 1. Panhandle N Region 2. Big Bend Region 3. Gulf Peninsula W E Region 4. Atlantic Peninsula Region 5. South Florida S Figure 8. Regions of Florida analyzed for scarred seagrasses. 20 FMRI Technical Report TR-1

31 F.J. Sargent et al Scarring of Florida s Seagrasses Gulf counties had the greatest acreages of both total and M/S scarring in this region. Scarring in this region was principally in the light category, although some of the small amount of seagrass in the more developed embayments had severe scarring. St. Joseph Bay, East Bay, and St. Andrew Bay, along with The Narrows and Santa Rosa Sound, were the principal foci for scarring. Big Lagoon in Escambia County had extensive light and moderate scarring, as did areas adjacent to Perdido Bay and Perdido Island. Region 2. Big Bend: The Big Bend region contains extensive areas of very shallow water and intricate shorelines. Even so, not much scarring was observed (Table 7). Within this region, Citrus County had the most extensive acreage of total scarring, and Pasco County had the most M/S scarring. Levy, Taylor, and Hernando counties also had a substantial amount of total scarring, most of which was in the light category. The extent of scarring was unexpected because of these counties lower population densities. However, the large amount of light scarring may have partially been an artifact of the small-scale maps that prevented detailed polygon delineation in this region. Region 3. Gulf Peninsula: The total acreage of scarred seagrasses was extensive in this region (Table 7). M/S scarring totaled 25,910 acres, which was the most in the state. Lee County had the most extensive total and M/S scarring of the counties in this region. The seagrass flats of Estero Bay, Pine Island Sound, and Matlacha Pass (all in Lee County) were criss-crossed with M/S scarring. Figure 9 illustrates detailed scarring patterns around Pine Island in Charlotte County. Note the scarred area to the southwest of the marina (lower left). Even though a marked boat channel (narrow band of light blue) extends west from the marina to open water and the Intracoastal Waterway, boats leaving the marina often take a shortcut south and as a result scar shallow seagrass beds. From Sarasota County to Pinellas County, light and moderate scarring were common. Pinellas County had the largest acreage of total and M/S scarring in the Tampa Bay region. The Gulf Peninsula region contains two extensive bay systems: Tampa Bay, which is highly developed, and Charlotte Harbor, which is much less developed. A comparison of the two bay systems shows that both total and M/S scarring were approximately the same for the two embayments. Charlotte Harbor has approximately 23,000 more acres of seagrass than Tampa Bay does, so scarring may have been more critical in Tampa Bay relative to its total seagrass acreage. Region 4. Atlantic Peninsula: This region had the lowest total acreage of scarred seagrasses (Table 7). Most scarring occurred in the northern part of Brevard County and the southern part of Volusia County, so the northern part of this region had the most extensive scarring. Within this region, Brevard County had the most total and M/S scarring, although Volusia County also had substantial M/S scarring. Relative to the total acreage of seagrass in the county, the scarring in Volusia County may be more deleterious. Counties south of Brevard County did not have substantial acreages of seagrass; therefore, scarring there was not extensive. Region 5. South Florida: This region has the largest acreage of seagrass in the state most of it in Monroe County (Table 7). This region also had the greatest acreage of severe scarring in the state. Monroe County had by far the most scarring in all categories in this region. Of the other counties in this region, Dade County had substantial scarring in the total and M/S categories, principally in southern Biscayne Bay. For this region, a better understanding of scarring can be obtained by viewing the Florida Keys as a single entity that crosses county boundaries. If the extensive area of seagrasses in Florida Bay is excluded from the scarring analysis, the Florida Keys contains what are probably the most egregious examples of scarring in the state. This area, which is in Dade and Monroe counties, provided a greater diversity of scarring types than any other county in the state and was surveyed in greater detail to provide an example of how to examine site-specific types of scarring (Kruer 1994). Virtually all seagrass banks and flats in the Florida Keys have some scarring, and scar density is generally greatest near developed islands and in areas of more intensive boating activity (Matthews et al. 1991). Scarred seagrasses were observed from the high intertidal zone to a depth of approximately six feet at low tide. The scars in deeper water were near ports at Key West and Stock Island; northeast of Big Pine Key, where commercial fish-trap boats take shortcuts through FMRI Technical Report TR-1 21

32 Scarring of Florida s Seagrasses F.J. Sargent et al Water Land Nonscarred Seagrass Light Scarring Moderate Scarring Severe Scarring SCALE 1:24,000 Scale 1:40,000 KILOMETERS MILES Pineland Marina Pine Island Part Island Alabama Georgia Gulf of Atlantic Ocean Mexico N W E S Figure 9. Detailed map of scarred seagrasses Pine Island, Lee County. 22 FMRI Technical Report TR-1

33 F.J. Sargent et al Scarring of Florida s Seagrasses SCALE 1:24,000 SCALE 1:40,000 KILOMETERS MILES Windley Key Alabama Georgia Gulf of Atlantic Ocean Light Scarring Moderate Scarring Severe Scarring W N Mexico E S Figure 10. Detailed map of scarred seagrasses Windley Key, Monroe County. FMRI Technical Report TR-1 23

34 Scarring of Florida s Seagrasses F.J. Sargent et al shallow channels; near Marathon and Islamorada, where large vessels dock; and in and along the Intracoastal Waterway on the Florida Bay side of the upper Florida Keys. Approximately 900 scarred areas were identified in the Florida Keys. Light scarring totaled 14,650 acres, moderate scarring totaled 10,400 acres, and severe scarring totaled 5,020 acres. The greatest concentration of M/S scarring was observed in the upper Keys. Scarring intensity ranged from a few scars at some sites to numerous propeller and grounding scars at others. Some formerly vegetated areas were covered by displaced sediment from extensively scarred and destabilized seagrass beds nearby. Moderately and severely scarred sites in the lower Florida Keys from the Marquesas Keys to near Snipe Key, for which additional information was collected, are listed in Tables 8 and 9 (Kruer 1994). These sites were evaluated for probable causes of scarring based on observed boating activity, environmental characteristics of the area, personal knowledge, and discussions with many boaters. A notable example of the intensity of scarring that occurs in some parts of Florida is around Windley Key, in the upper Florida Keys. Windley Key is in the Florida Keys National Marine Sanctuary, and its waters are designated as Class III Outstanding Florida Waters. It includes a variety of shallow marine communities and is a transitional area between high-energy oceanic waters and the more protected waters of Florida Bay and Everglades National Park. Whale Harbor and Snake Creek channels, both with relatively deep water, connect the extensive, shallow seagrass flats of Florida Bay with deeper oceanic waters. Endangered West Indian manatees, American crocodiles, and seaturtles are known to inhabit these waters. The area is also surrounded by coral reefs and hardbottom communities that attract many tourists and fishermen. As a result of intense boating activity and lack of proper protection, the Windley Key area contains some of the most heavily scarred seagrass flats in south Florida. Figure 10 illustrates the extent of seagrass scarring around Windley Key. Kruer (1994) noted the loss of seagrasses along channel edges and that eroded sediments were being deposited on seagrasses. Unmarked channels had been cut through shallow-water seagrass flats and between mangroves. Boat wakes severely eroded seagrass beds along the offshore channel edges. Boating activity originated from facilities located at Whale Harbor Channel and at Snake Creek and from the more than 31,000 linear feet of bulkhead docks along canals in a residential subdivision. Scarring of seagrasses in the Florida Keys has occurred for some time probably since combustion engines (outboard and inboard) were installed in boats. However, the problem of seagrass scarring has become acute because of the increasing residential population; the increasing popularity of boating, fishing, diving, and other water sports; and increasing tourism. New propdredged channels continue to appear, some thousands of feet long, providing access for larger and more numerous vessels into areas not previously accessible. Many shallow flats and banks are now severely eroded due to constant scarrings, ship groundings, chronic wave action, and water-current scouring (Kruer 1994). Discussion The majority of Florida s moderate/severe (M/S) seagrass scarring (68.9 percent) occurred in five counties: Monroe, Dade, Lee, Charlotte, and Pinellas. These same counties contain 63.4 percent of the state s seagrass acreage. However, if Monroe County is excluded from analysis because of its disproportionately large amount of seagrass acreage and scarring the five counties that have the most M/S scarring contain only 9.3 percent of the state s seagrass acreage but 49.4 percent of its M/S scarring. What could be the cause of so much scarring in these counties? One important correlation exists with population density, as reflected in vessel registrations for each of the counties. For example, M/S scarring of seagrass beds is greater in the densely populated Gulf Peninsula region than it is in the sparsely populated Big Bend region. Florida s population nearly doubled between 1970 and 1990: from 6,791,000 to 12,938,000. During the same period, the number of vessel registrations (recreational and commercial) more 24 FMRI Technical Report TR-1

35 F.J. Sargent et al Scarring of Florida s Seagrasses Table 8. Moderately scarred sites Marquesas Keys to Snipe Key ( ). Adapted from Kruer (1994). Site # Adjacent Key Probable Suggested Cause 1 Management 2 Comments 13 Marquesas Keys A, S ED Shallow channel between islands with popular beaches 15 Marquesas Keys A, S ED Shallow channel between two islands 32 Marquesas Keys I, S ED Entrance to natural channel 46 Boca Grande Key C, S C At entrance to main channel, existing markers (#17 and #18) on chart, marker 18 in shallow zone, vessels pass on shallow side. 50 Boca Grande Key C, S C Markers # 13 and 14 not located as shown on chart Archer Key C, S C, EN Adjacent to single marker # 8 shown on chart 11441, oversized vessels, needs gated markers. 105 Mule Key C, S C, EN Confined area between markers, used by oversized vessels 113 Mule Key C, S ED Area of concentrated traffic near channel markers 121 Key West S ED Isolated bank (Middle Grounds) in center of Northwest Channel 123 Wisteria Island C, L, S C, ED, EN Heavily traveled anchorage on west edge of Key West Channel 127 Fleming Key C, S C, ED Inadequately marked channel through large bank 142 Fleming Key S ED, EN On inside of several markers 145 Fleming Key S C, ED On edge of main channel near marker 150 Key West P, S C, ED, EN Outside of markers in access to Garrison Bight 151 Key West C, P, S C, ED Inside Garrison Bight, outside of partly marked dredged channel 152 Key West C, P, S C, ED Inside Garrison Bight, outside of partly marked dredged channel 155 Sigsbee Park S C, ED At end of dredged area 156 Key West A, S C, ED Boats accessing dredged channel 157 Key West C, P, S C, ED, EN Cow Key Channel, part marked, part unmarked, high-speed traffic 163 Stock Island P, S C, ED, EN Adjacent to Safe Harbor Channel 165 Stock Island L, P, S C, ED, EN Anchorage east of Stock Island in Boca Chica Channel 166 Stock Island L, P ED, EN Anchorage east of Stock Island in Boca Chica Channel and near ramp 174 Boca Chica S ED, EN At entrance to dredged part of Boca Chica Channel 181 Bay Keys I, S ED, EN Commercial tour boats and recreational boats accessing 201 Lower Harbor Bay Keys from the south Keys I, S ED, EN Long, illegally marked channel 204 Channel Key I, S ED Part of old Backcountry Waterway 207 Channel Key I, S C, ED Cut through bank between islands 223 Fish Hawk Key I, S C, ED Cut through long linear bank 232 Geiger Key A, I, S ED, EN Shallow channel leaving residential canal 236 Saddlebunch Key C,S C, ED On bank near marked channel 238 Big Coppitt Key A,C ED, EN Marked access to canal trailer park 245 Halfmoon Key A, I, S ED, EN Access to shallow embayment 251 Mud Keys S C, ED Channel leaving Waltz Key Basin 1 Probable Cause: A = access point, C = poor channel markers, I = illegal aids to navigation, L = live-aboards, P = proximity, S = shortcut. 2 Suggested Management: C = new or improved markers, ED = education, EN = better enforcement, R = restricted area. FMRI Technical Report TR-1 25

36 Scarring of Florida s Seagrasses F.J. Sargent et al Table 9. Severely scarred sites Marquesas Keys to Snipe Key ( ). Adapted from Kruer (1994). Site # Adjacent Key Probable Suggested Cause 1 Management 2 Comments 7 Marquesas Keys S ED, EN From large vessel in early 1980s, now enlarged 129 Wisteria Island C, L, S C, ED, EN Heavily traveled anchorage on east side of Key West Channel 138 Fleming Key I, S C, ED At shallow end of a natural channel 158 Stock Island A ED, EN Boats accessing residential area in shallow water 160 Key West A, L ED, EN Cow Key Channel live-aboard anchorage and Cow Key Channel south of bridge 170 Stock Island S C, ED, EN Large vessels shortcutting into Boca Chica Channel 231 Geiger Key A, I, P, S C, ED, EN Access to Geiger Key Marina and area 1 Probable Cause: A = access point, C = poor channel markers, I = illegal aids to navigation, L = live-aboards, P = proximity, S = shortcut. 2 Suggested Management: C = new or improved markers, ED = education, EN = better enforcement, R = restricted area. than tripled: from 235,000 to 716,000. Clearly, not only is the population increasing, but the percentage of the population that owns boats is also increasing. Substantial increases in both population and number of vessels suggest that our state s water resources are being used at an increasing rate, and therefore its seagrasses are in increasing danger of being damaged. By , total power-boat registration for the 31 counties in this survey had reached 493,406 vessels (Bureau of Vessel Titles and Registrations 1994). The greatest percentage of boats in most coastal counties were registered as pleasure boats (Table 10). For the 31 counties in this study, only 6.4 percent of vessels were registered as commercial craft. The five counties with the greatest number of vessel registrations were Dade, Pinellas, Broward, Hillsborough, and Lee. These five counties contained 40.6 percent of all vessels registered in the 31 coastal counties in this study. The number of registered vessels in the five counties with the greatest acreage of M/S scarring was 156,899 in , which is 14 times greater than that of the registered vessels in the five counties that had the least M/S scarring (11,031 acres) and that also had substantial seagrass acreage. In four of the five counties with the most registered craft, M/S scarring of seagrasses was also extensive (25,160 acres or 39 percent). In Broward County, scarring levels were low because it had only slightly more than one acre of seagrasses (based on small-scale photography). The number of vessels registered in a county is not always a predictor of seagrass scarring in that county. Many counties with large numbers of registered watercraft lack substantial seagrass acreage. For example, Palm Beach County has 30,929 and Broward County has 42,612 registered vessels (Table 10), but each has less than 20 acres of M/S scarring (Table 2). In contrast, Monroe County has a moderate number of vessels registered (20,163) but contains the greatest acreage of M/S scarring in the state. Whether a vessel is used for commercial or recreational purposes may influence where it is predominantly used. Commercial vessels are usually larger, work farther offshore, and are limited to a few ports with deeper access; smaller vessels can be trailered to attractive inshore fishing and watersports areas such as the Florida Keys and Charlotte Harbor. Pleasure boats (excluding sailboats) in most counties compose more than 90 percent of registered vessels (Table 10). In Monroe County, by contrast, only 80 percent (16,152) of the vessels are registered as pleasure boats; the remainder are registered as commercial vessels. Pleasure-boat registrations indicate where trailering may likely originate. Therefore, seagrass scarring in the Florida Keys may be caused in part by smaller boats trailered in from Palm Beach, Dade, and Broward counties and elsewhere. Nevertheless, seagrass scarring is not limited to a single group of boaters; all user-groups scar seagrasses to some degree. 26 FMRI Technical Report TR-1

37 F.J. Sargent et al Scarring of Florida s Seagrasses Table 10. Vessel registrations in in the 31 Florida coastal counties in this study. Table does not include sailboat registrations. County Pleasure Craft Percentage Pleasure Craft Commercial and Dealer Craft Total Watercraft County Rank BAY 13, ,488 14, BREVARD 25, ,716 27,479 7 BROWARD 39, ,682 42,612 3 CHARLOTTE 14, , CITRUS 11, ,039 12, COLLIER 13, ,171 14, DADE 44, ,231 46,773 1 DIXIE 1, , ESCAMBIA 15, , FRANKLIN 1, ,045 2, GULF 1, , HERNANDO 5, , HILLSBOROUGH 35, ,099 4 INDIAN RIVER 7, , JEFFERSON LEE 29, ,007 31,416 5 LEVY 2, , MANATEE 12, , MARTIN 12, , MONROE 16, ,011 20,163 8 OKALOOSA 13, , PALM BEACH 29, ,067 30,929 6 PASCO 14, , PINELLAS 41, ,279 43,596 2 SANTA ROSA 7, , SARASOTA 16, , ST. LUCIE 8, , TAYLOR 2, , VOLUSIA 18, ,162 9 WAKULLA 3, , WALTON 2, , FMRI Technical Report TR-1 27

38 Scarring of Florida s Seagrasses F.J. Sargent et al Table 11. Vessel registrations in (Bureau of Vessel Titles and Registrations 1994) in the five regions of Florida demarcated in this survey (see Figure 8). Table does not include sailboat registrations. Region Total Pleasure % Pleasure Regional Rank Watercraft Craft (PC) Craft PC %PC 1. PANHANDLE 59,156 54, BIG BEND 44,893 41, GULF PENINSULA 156, , ATLANTIC PENINSULA 108, , SOUTH FLORIDA 124, , TOTAL 493, , On a regional basis, vessel registrations were greatest in the Gulf Peninsula region (Table 11). Vessel registrations in the Panhandle and Big Bend regions were insignificant compared to those in the other three regions. The Gulf Peninsula region not only had the greatest number of registered vessels, it also had the greatest percentage registered as pleasure craft (95 percent) and the most M/S scarring. The South Florida region was second in the number of registered vessels and nearly equal to the Gulf Peninsula region in M/S scarring. The Gulf Peninsula and South Florida regions contained 57 percent of all registered vessels in the 31 coastal counties in this study. The lowest number of registered vessels was in the Big Bend region (9.1 percent). Many authors have speculated on the situations in which seagrasses are scarred (e.g., Woodburn et al. 1957, Godcharles 1971, Eleuterius 1987, Zieman and Zieman 1989, Wilderness Society et al. 1990). Our discussions with boaters, as well as our personal experiences, suggest that scarring of seagrasses could result when one or more of the following situations occur: when boaters misjudge water depth and accidentally scar seagrass beds; when boaters who lack navigational charts or the skill to use them stray from poorly marked channels and accidentally scar seagrass beds; when boaters intentionally leave marked channels to take shortcuts through shallow seagrass beds, knowing that seagrasses may be scarred; when boaters carelessly navigate in shallow seagrass beds because they believe scars heal quickly; when inexperienced boaters engage in recreational and commercial fishing in shallow seagrass flats, thinking that their boat s designed draft is not deep enough to scar seagrasses or that the design will prevent damage to their boat; when boaters overload their vessels, causing deeper drafts than the boaters realize; when boaters anchor over shallow seagrass beds, where their boats swing at anchor and scar seagrasses; when boaters intentionally prop-dredge to create a channel; and when inexperienced boaters, ignorant of what seagrasses are and the benefits they provide, accept as the behavioral norm local boating customs that disregard the environment. The situations that promote scarring can be grouped into two general categories: (1) All too often, boaters accidentally or intentionally pass through water that is too shallow for the draft of their vessels. The average size, draft, and power of vessels are increasing; therefore, bigger, more powerful vessels are being navigated through shallow waters and are scarring more seagrass acreage. Also, water sports often occur in shallow water, although suitable deeper water may be found nearby. Boaters use flats boats, which are designed to operate in shallow water, to gain access to more remote, shallow seagrass beds. However, inexperi- 28 FMRI Technical Report TR-1

39 F.J. Sargent et al Scarring of Florida s Seagrasses enced users of flats boats, ignorant of the proper use of such boats and the great value of seagrasses, may extensively scar shallow seagrass beds in areas near marinas and launching ramps. Inexperienced boaters, unfamiliar with the location of channels and often lacking navigational charts, travel through areas where official channel markers are infrequent or poorly located. Some channel markers are not located as shown on charts, and many are immediately adjacent to shallow-water seagrasses. Furthermore, many boaters are unfamiliar with the meanings of U.S. Coast Guard Aids to Navigation; hence, a single marker may confuse an inexperienced boater who is unable to read either the marker or water depth. Running aground is more likely if a boater passes on the wrong side of a marker located on the edge of a seagrass flat. Markers in some channels do not extend an adequate distance beyond the ends of channels to discourage boaters from crossing the edges of seagrass beds. Notably, many M/Sscarred seagrasses are in or adjacent to the ends of officially marked channels. Illegal aids to navigation (e.g., PVC, wood, or metal posts and marker buoys) are widespread, especially in the Florida Keys. Only those who place these markers know what is intended. Often these illegal markers indicate where prop-dredging has deepened shallow areas so small boats can get between deeper areas. Boaters in larger vessels may attempt to use such markers and unexpectedly pass through water too shallow for their boats. (2) Coastal property is popular because it allows direct access to the water. Extensive shoreline development in shallow bays and adjacent to shallow seagrass flats results in increased scarring. Some seagrass scarring is caused by boaters who attempt to gain access to shoreline development and by coastal landowners and their families and friends boating in nearby shallows. Many dredged canals leading from residential areas terminate in relatively shallow water (Figure 11). Current state and county rules in many areas limit new docks to waters greater than a specific depth at low tide, but many old docks are located in shallow water and have poorly defined access channels, if they have them at all. Many older access channels in open water are subject to sedimentation and are maintained by prop-dredging. Development is not just restricted to uplands. The number of live-aboard vessels has increased in some areas. Scarring of seagrasses by hulls, anchors, and chains occurs as live-aboard vessels (and other boats) swing at anchor over shallow seagrass beds (Kruer 1994). The increasing number of access points such as boat launching ramps has also contributed to seagrass scarring. Boating-access areas are usually located in sheltered areas, where seagrasses are more likely to occur. Hundreds of commercial marinas, watercraft rentals, and public boat ramps are near shallow seagrass beds where few channel markers exist. Because these channels are usually subject to heavy sedimentation, regular dredging is often needed to keep access open. Some of the most severely scarred areas in Florida are near marinas catering to flats fishermen in the Florida Keys. Management Options When state funds for seagrass management are limited, the money should be invested in those counties that have the greatest acreage of M/Sscarred seagrasses (e.g., Monroe County). However, if the severity of seagrass-habitat loss in a county is related to the extent of seagrasses in that county, then counties with both moderate seagrass acreage and more intense scarring may merit similar attention when management programs are being developed. Based on a scarring index (SI) in which the relative percentage of M/S scarring in a county is divided by the relative percentage of total seagrass acreage for that county (Table 3), the more threatened counties are Hillsborough (19.0), Charlotte (18.4), Collier (13.0), Pinellas (10.2), Manatee (9.4), Volusia (7.0), and Lee (6.9). When assigning management priorities, however, other aspects of scarring extent must also be considered. Because extensive areas of seagrasses are in water depths greater than six feet, where they are unlikely to be scarred, including these acreages in SI calculations lessens the apparent extent of scarring in some counties. If deeper seagrass beds are excluded from the SI calculations, then county rankings would be considerably different. For example, Monroe County which has a high degree of M/S scarring would rank low using only an SI value because of the county s FMRI Technical Report TR-1 29

40 Scarring of Florida s Seagrasses F.J. Sargent et al Figure 11. Example of a channel serving a residential area and ending in a shallow seagrass bed. 30 FMRI Technical Report TR-1